It is known that polymers bearing ionic groups may be used for producing membranes for fuel cells. The ionic groups may be acid groups or alkaline groups.
It is also known how to prepare polymer films either by casting from a solution of the polymer in a volatile solvent, or by extrusion. Extrusion is advantageous, since it makes it possible to avoid the use of volatile solvents, which may be flammable.
A polymer may be extruded on condition that it has a thermal stability such that the polymer is not degraded at the temperature required for the extrusion, which depends on the glass transition temperature.
The comparison of the thermal stability of a polymer having a given backbone that does not bear ionic groups with the thermal stability of a polymer which has the same backbone but which bears ionic groups shows that the polymer with ionic groups has a lower thermal stability. The degradation temperature is therefore lower and it is generally incompatible with the temperature required for extrusion, which depends on the glass transition temperature.
It is known to lower the glass transition temperature of a polymer by mixing it with a plasticizer, to enable extrusion without thermal degradation, the plasticizer then being removed after extrusion. The polymers that have been extruded by this process are polymers which do not bear ionic groups. As plasticizers used for the extrusion of thermoplastic polymers, mention may be made of chlorinated or non-chlorinated paraffins, carboxylic esters (such as adipates, benzoates, citrates and phthalates), phosphoric esters, and toluene disulfonamides. For example, H. H. Kausch, et al., (Traité des Matéeriaux (Materials Compendium), volume 14, Presses Polytechniques et Universitaires Romandes, Lausanne, 2001) describe the extrusion of polystyrene after incorporating organic agents as plasticizer.
The inventors have then envisaged preparing films of polymers bearing acid ionic groups by subjecting a polymer and plasticizer mixture to extrusion, then by removing the plasticizer from the film obtained by extrusion. However, it has turned out that, among the many compounds known as plasticizers for extruding polymers without ionic groups, all were unable to be used with a polymer having acid groups. For example, the paraffins had little affinity for the ionic groups. In addition, in the presence of small amounts of moisture, the carboxylic esters and the phosphoric esters are degraded by the acid ionic groups of the polymer to be extruded.
The work of the inventors has thus enabled them to define a family of compounds that can be used as plasticizers for polymers bearing acid ionic groups, said plasticizers lowering the glass transition temperature and allowing an extrusion temperature that remains below the degradation temperature of the polymer.